Three WASP-South Transiting Exoplanets: WASP-74b, WASP-83b, and WASP-89b

arXiv:1410.6358v2 [astro-ph.EP] 24 Oct 2014

Three WASP-South transiting exoplanets:

WASP-74b, WASP-83b & WASP-89b

Coel Hellier

_{, D.R. Anderson}

_{, A. Collier Cameron}

_{, L. Delrez}

_{, M. Gillon}

_{, E. Jehin}

_{, M.}

Lendl

_{, P.F.L. Maxted}

_{, F. Pepe}

_{, D. Pollacco}

_{, D. Queloz}

_{, D. S´egransan}

_{, B. Smalley}

_,

A.M.S. Smith

_{, J. Southworth}

_{, A.H.M.J. Triaud}

_{, O.D. Turner}

_{, S. Udry}

_,

& R.G. West

ABSTRACT

We report the discovery of three new transiting hot Jupiters by WASP-South together with the TRAPPIST photometer and the Euler/CORALIE spectrograph.

WASP-74b orbits a star of V = 9.7, making it one of the brighter systems accessible to Southern telescopes. It is a 0.95 MJup planet with a moderately bloated radius of 1.5 RJup in a

2-d orbit around a slightly evolved F9 star.

WASP-83b is a Saturn-mass planet at 0.3 MJup with a radius of 1.0 RJup. It is in a 5-d orbit

around a fainter (V = 12.9) G8 star.

WASP-89b is a 6 MJup planet in a 3-d orbit with an eccentricity of e = 0.2. It is thus similar

to massive, eccentric planets such as XO-3b and HAT-P-2b, except that those planets orbit F stars whereas WASP-89 is a K star. The V = 13.1 host star is magnetically active, showing a rotation period of 20.2 d, while star spots are visible in the transits. There are indications that the planet’s orbit is aligned with the stellar spin. WASP-89 is a good target for an extensive study of transits of star spots.

Subject headings: planetary systems — stars: individual (WASP-74, WASP-83, WASP-89) 1. Introduction

The combination of the WASP-South survey in-strument, the Euler/CORALIE spectrograph and 1_{Astrophysics Group, Keele University, Staffordshire,}

ST5 5BG, UK

2_{SUPA, School of Physics and Astronomy, University of}

St. Andrews, North Haugh, Fife, KY16 9SS, UK

3_{Institut d’Astrophysique et de G´eophysique,}

Univer-sit´e, Li`ege, All´ee du 6 Aoˆut, 17, Bat. B5C, Li`ege 1, Bel-gium

4

Observatoire astronomique de l’Universit´e de Gen`e, 51 ch. des Maillettes, 1290 Sauverny, Switzerland

5_{Department of Physics, University of Warwick, Gibbet}

Hill Road, Coventry CV4 7AL, UK

6_{Cavendish Laboratory, J J Thomson Avenue,}

Cam-bridge, CB3 0HE, UK

7

N. Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland

8_{Department of Physics and Kavli Institute for}

Astro-physics & Space Research, Massachusetts Institute of Tech-nology, Cambridge, MA 02139, USA

the robotic TRAPPIST photometer continue to be an efficient team for the discovery of transiting exoplanets around stars of V = 9–13 in the South-ern Hemisphere (e.g. Hellier et al. 2014; Ander-son et al. 2014a). Ongoing discoveries are impor-tant for expanding our census of the hot-Jupiter population, while exoplanets transiting relatively bright stars are good targets for followup stud-ies. In this paper we report three new discoveries: WASP-74b, which orbits a bright V = 9.7 star; WASP-83b, a more moderately bloated Saturn-mass planet, which, with a period of 4.97 d demon-strates the capability of a single-longitude tran-sit search to find planets with integer-day periods; and WASP-89b, a massive planet in a short and ec-centric orbit around a magnetically active K star.

The observational and analysis techniques used here are similar to those in recent WASP-South discovery papers (e.g. Hellier et al. 2012; Anderson et al. 2014b), and so are reported briefly. WASP-South surveys the WASP-Southern sky using an array of 200mm f/1.8 lenses and a cadence of ∼ 10 mins (see Pollacco et al. 2006). Transit search-ing of accumulated lightcurves (Collier-Cameron et al. 2007a) leads to tens of thousands of possible candidates, of which the vast majority are false alarms resulting from the limitations of the pho-tometry. The best 1 per cent are selected by eye as candidates and are passed to TRAPPIST (a robotic 0.6-m photometric telescope) and to the 1.2-m Euler/CORALIE spectrograph (for radial-velocity observations). About 1 in 12 candidates turns out to be a planet, with most of the oth-ers being astrophysical transit mimics (blended or grazing eclipsing binaries). Higher-quality tran-sit lightcurves are then obtained with TRAPPIST (Jehin et al. 2011) and with EulerCAM (Lendl et al. 2012). We have also observed a transit of WASP-74b using RISE on the Liverpool Telescope (see Steele et al. 2008).

A list of the observations reported here is given in Table 1 while the CORALIE radial velocities are listed in Table A1.

3. The host stars

We used the CORALIE spectra to analyse the three host stars, co-adding the standard pipeline reduction products to produce spectra with S/N ratios of 150:1, 100:1 and 30:1 for WASP-74, WASP-83 and WASP-89 respectively. Our anal-ysis methods are described in Doyle et al. (2013). The effective temperature (Teff) estimate comes

from the excitation balance of Fe i lines, while the surface gravity (log g) estimates comes from the ionisation balance of Fe i and Fe ii and the Ca i line at 6439˚A and the Na i D lines. The metal-licity was determined from equivalent width mea-surements of several unblended lines. The quoted error estimates include that given by the uncer-tainties in Teff and log g, as well as the scatter due

to measurement and atomic data uncertainties. The projected stellar rotation velocity (v sin i) was determined by fitting the profiles of several unblended Fe i lines. Values of macroturbulent

Facility Date

WASP-74:

WASP-South 2010 May–2012 Jun 10 000 points

CORALIE 2011 Aug–2012 Oct 20 RVs

EulerCAM 2012 May 07 Gunn r filter

TRAPPIST 2012 May 07 z′ _band

EulerCAM 2012 May 22 Gunn r filter

TRAPPIST 2012 May 22 z′ _band

TRAPPIST 2012 Jun 21 z′ _band

TRAPPIST 2012 Jun 23 z′ _band

TRAPPIST 2012 Sep 04 z′ _band

TRAPPIST 2013 Jun 27 I+ z band

LT/RISE 2014 Aug 19 V + R

WASP-83:

WASP-South 2006 May–2010 Jun 20 600 points

CORALIE 2011 Mar–2013 Feb 28 RVs

TRAPPIST 2012 Jan 07 Blue-block filter TRAPPIST 2012 Jan 22 Blue-block filter TRAPPIST 2012 Feb 06 Blue-block filter WASP-89:

WASP-South 2008 Jun–2012 Jun 18 000 points

CORALIE 2011 May–2013 May 20 RVs

TRAPPIST 2012 Aug 26 Blue-block filter

EulerCAM 2012 Sep 12 Gunn r filter

TRAPPIST 2012 Sep 12 Blue-block filter

EulerCAM 2012 Oct 02 Gunn r filter

TRAPPIST 2012 Oct 02 Blue-block filter TRAPPIST 2013 Jun 14 Blue-block filter TRAPPIST 2013 Aug 27 Blue-block filter

velocity of 3.9 ± 0.7 km s−1_{and 2.9 ± 0.7 km s}−1

were adopted for WASP-74 and WASP-83, using the calibration of Doyle et al. (2014). For WASP-89, however, macroturbulence was assumed to be zero, since for mid-K stars it is expected to be lower than that of thermal broadening (Gray 2008).

The parameters obtained from the analysis are given in Tables 2 to 4. The quoted spectral type derives from Teff, using the values in Gray (2008).

Abundances are relative to the solar values ob-tained by Asplund et al. (2009). Gyrochronologi-cal age estimates derive from the measured v sin i, assuming that the star’s spin is perpendicular to us, so that this is the true equatorial speed. This is then combined with the stellar radius to give a rotational period, to compare with the values

in Barnes (2007). Lithium age estimates come from values in Sestito & Randich (2005). We also list proper motions from the UCAC4 catalogue of Zacharias et al. (2013).

We searched the WASP photometry of each star for rotational modulations by using a sine-wave fitting algorithm as described by Maxted et al. (2011). We estimated the significance of period-icities by subtracting the fitted transit lightcurve and then repeatedly and randomly permuting the nights of observation. We found a significant mod-ulation in WASP-89 (see Section 8.1) but not in the other two stars.

4. System parameters

The CORALIE radial-velocity measurements were combined with the WASP, EulerCAM and TRAPPIST photometry in a simultaneous Markov-chain Monte-Carlo (MCMC) analysis to find the system parameters. For details of our methods see Collier Cameron et al. (2007b). The limb-darkening parameters are noted in each Table, and are taken from the 4-parameter non-linear law of Claret (2000).

For WASP-89b the orbital eccentricity is sig-nificant and was fitted as a free parameter. For WASP-74b and WASP-83b we imposed a circular orbit during the analysis (see Anderson et al. 2012 for the rationale for this).

The fitted parameters were Tc, P , ∆F , T14, b,

K1, where Tc is the epoch of mid-transit, P is

the orbital period, ∆F is the fractional flux-deficit that would be observed during transit in the ab-sence of limb-darkening, T14 is the total transit

duration (from first to fourth contact), b is the impact parameter of the planet’s path across the stellar disc, and K1 is the stellar reflex velocity

semi-amplitude. The transit lightcurves lead di-rectly to stellar density but one additional con-straint is required to obtain stellar masses and radii, and hence full parametrisation of the sys-tem. Here we use the calibrations presented by Southworth (2011), based on masses, radii and ef-fective temperatures of eclipsing binaries.

For each system we list the resulting parameters in Tables 2 to 4, and plot the resulting data and models in Figures 1 to 4. We also refer the reader to Smith et al. (2012) who present an extensive analysis of the effect of red noise in the transit

Table 2: System parameters for WASP-74. 1SWASP J201809.32–010432.6

2MASS 20180931–0104324

RA = 20h₁₈m_09.32s_{, Dec = –01}◦₀₄′

32.6′′ (J2000) V mag = 9.7

Rotational modulation < 0.7 mmag (95%) pm (RA) 1.6 ± 1.0 (Dec) –64.3 ± 0.7 mas/yr Stellar parameters from spectroscopic analysis.

Spectral type F9 Teff (K) 5990 ± 110 log g 4.39 ± 0.07 v sin I (km s−1_{) 4.1 ± 0.8} [Fe/H] +0.39 ± 0.13 log A(Li) 2.74 ± 0.09

Age (Lithium) [Gy] 0.5 ∼ 2

Age (Gyro) [Gy] 2.0+1.6−1.0

Distance [pc] 120 ± 20

Parameters from MCMC analysis.

P (d) 2.137750 ± 0.000001 Tc (HJD) (UTC) 245 6506.8918 ± 0.0002 T14 (d) 0.0955 ± 0.0008 T12= T34 (d) 0.0288 ± 0.0014 ∆F = R2 P/R 2 ∗ 0.00961 ± 0.00014 b 0.860 ± 0.006 i(◦_{) 79.81 ± 0.24} K1 (km s−1) 0.1141 ± 0.0014 γ (km s−1_{) –15.767 ± 0.001} e 0 (adopted) (< 0.07 at 3σ) M∗ (M⊙) 1.48 ± 0.12 R∗ (R⊙) 1.64 ± 0.05 log g∗ (cgs) 4.180 ± 0.018 ρ∗ (ρ⊙) 0.338 ± 0.018 Teff (K) 5970 ± 110 MP (MJup) 0.95 ± 0.06 RP(RJup) 1.56 ± 0.06 log gP (cgs) 2.95 ± 0.02 ρP (ρJ) 0.25 ± 0.02 a(AU) 0.037 ± 0.001 TP,A=0(K) 1910 ± 40

Errors are 1σ; Limb-darkening coefficients were: Trap z: a1 = 0.757, a2 = –0.591, a3 = 0.890, a4 = –0.416

Euler & RISE: a1 = 0.669 , a2 = –0.284 , a3 = 0.765, a4 = –0.395

0 0.06 0.6 0.8 1 1.2 1.4 Rel. mag 0.86 0.87 0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 0.96 0.98 1 1.02 1.04 Relative flux WASP EulerCAM TRAPPIST EulerCAM TRAPPIST TRAPPIST TRAPPIST TRAPPIST TRAPPIST RISE

Fig. 1.— WASP-74b discovery photometry: (Top) The WASP data folded on the transit period. (Second panel) The binned WASP data with (off-set) the follow-up transit lightcurves (ordered from the top as in Table 1) together with the fitted MCMC model. The two EulerCAM lightcurves are of the same transit as the TRAPPIST data directly below them.

-100 0 100 Rel. RV (m s -1 ) CORALIE -40 -20 0 20 40 RV resids (m s -1 ) -40 -20 0 20 40 0.6 0.8 1 1.2 1.4 Bis. span Orbital phase

Fig. 2.— WASP-74b radial velocities and fitted model (top) along with (middle) the residuals and (bottom) the bisector spans; the absence of any correlation with radial velocity is a check against transit mimics.

Table 3: System parameters for WASP-83. 1SWASP J124036.51–191703.4

2MASS 12403650–1917032

RA = 12h₄₀m_36.51s_{, Dec = –19}◦₁₇′_03.4′′ _(J2000)

V mag = 12.9

Rotational modulation < 1.5 mmag (95%) pm (RA) –21.6 ± 1.9 (Dec) –11.5 ± 2.1 mas/yr Stellar parameters from spectroscopic analysis.

Spectral type G8 Teff (K) 5480 ± 110 log g 4.34 ± 0.08 v sin I (km s−1₎ <0.5 [Fe/H] +0.29 ± 0.12 log A(Li) <0.75

Age (Lithium) [Gy] &5

Distance [pc] 300 ± 50

Parameters from MCMC analysis.

P (d) 4.971252 ± 0.000015 Tc (HJD) (UTC) 245 5928.8853 ± 0.0004 T14 (d) 0.1402 ± 0.0015 T12= T34 (d) 0.0136 ± 0.0017 ∆F = R2 P/R2∗ 0.0104 ± 0.0004 b 0.23 ± 0.15 i(◦_{) 88.9 ± 0.7} K1 (km s−1) 0.0329 ± 0.0031 γ (km s−1_{) 31.549 ± 0.002} e 0 (adopted) (< 0.3 at 3σ) M∗ (M⊙) 1.11 ± 0.09 R∗ (R⊙) 1.05+0.06_−0.04 log g∗ (cgs) 4.44+0.02−0.04 ρ∗ (ρ⊙) 0.97+0.07_−0.13 Teff (K) 5510 ± 110 MP (MJup) 0.30 ± 0.03 RP(RJup) 1.04+0.08−0.05 log gP (cgs) 2.79 ± 0.06 ρP (ρJ) 0.26 ± 0.05 a(AU) 0.059 ± 0.001 TP,A=0(K) 1120 ± 30

Errors are 1σ; Limb-darkening coefficients were: a1 = 0.747, a2 = –0.649, a3 = 1.277, a4 = –0.584 -0.06 0 0.06 0.6 0.8 1 1.2 1.4 Rel. mag 0.94 0.95 0.96 0.97 0.98 0.99 1 1.01 0.96 0.98 1 1.02 1.04 Relative flux WASP TRAPPIST TRAPPIST TRAPPIST -60 -30 0 30 60 Rel. RV (m s -1 ) CORALIE -60 -30 0 30 60 0.6 0.8 1 1.2 1.4 Bis. span Orbital phase

Fig. 3.— WASP-83b discovery data, as for Figs. 1 and 2.

Table 4: System parameters for WASP-89. 1SWASP J205535.98–185816.1 2MASS 20553599–1858159 RA = 20h₅₅m_35.98s_{, Dec = –18}◦₅₈′ 16.1′′ (J2000) V mag = 13.1

pm (RA) 13.7 ± 1.5 (Dec) –63.0 ± 1.4 mas/yr Stellar parameters from spectroscopic analysis.

Spectral type K3 Teff (K) 4955 ± 100 log g 4.31 ± 0.16 v sin I (km s−1_{) 2.5 ± 0.9} [Fe/H] +0.15 ± 0.14 log A(Li) <0.24

Age (Lithium) [Gy] &0.5 Age (Gyro) [Gy] 1.3+1.5−0.8

Parameters from MCMC analysis.

P (d) 3.3564227 ± 0.0000025 Tc (HJD) (UTC) 245 6207.02114 ± 0.00012 T14 (d) 0.1025 ± 0.0004 T12= T34 (d) 0.0112 ± 0.0003 ∆F = R2 P/R 2 ∗ 0.0149 ± 0.0002 b 0.10 ± 0.08 i(◦_{) 89.4 ± 0.5} K1 (km s−1) 0.848 ± 0.013 γ (km s−1_{) 21.088 ± 0.008} e 0.193 ± 0.009 ω (deg) 28 ± 4 M∗ (M⊙) 0.92 ± 0.08 R∗ (R⊙) 0.88 ± 0.03 log g∗ (cgs) 4.515 ± 0.018 ρ∗ (ρ⊙) 1.36 ±0.07 Teff (K) 5130 ± 90 MP (MJup) 5.9 ± 0.4 RP(RJup) 1.04 ± 0.04 log gP (cgs) 4.10 ± 0.02 ρP (ρJ) 5.27 ± 0.33 a(AU) 0.0427 ± 0.0012 TP,A=0(K) 1120 ± 20

Errors are 1σ; Limb-darkening coefficients were: a1 = 0.741, a2 = –0.739, a3 = 1.427, a4 = –0.620 0 0.1 0.6 0.8 1 1.2 1.4 Rel. mag 0.87 0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 0.98 1 1.02 Relative flux Orbital phase WASP TRAPPIST EulerCAM 12 Sep 2012 TRAPPIST EulerCAM 02 Oct 2012 TRAPPIST TRAPPIST TRAPPIST

Fig. 4.— WASP-89b discovery data. The Euler and TRAPPIST observations on 12 Sep 2012 and on 02 Oct 2012 are of the same transit, and hence these transits are plotted slightly closer together. Otherwise the lightcurves are as for Fig. 1 6

-800 -400 0 400 800 1200 Rel. RV (m s -1 ) CORALIE -200 -100 0 100 200 RV resids (m s -1 ) -400 0 400 0.6 0.8 1 1.2 1.4 Bis. span Orbital phase

Fig. 5.— WASP-89b radial velocities (as for Fig. 2).

lightcurves on the resulting system parameters. 5. Evolutionary status

One area where the methods of this paper dif-fer from those of previous WASP-South discover-ies is in the comparison of the stellar parameters to evolutionary models. Here we use the method described in detail in Maxted, Serenelli & South-worth (2014). This uses an MCMC method to calculate the posterior distribution for the mass and age estimates of the star, by comparing the observed values of ρ⋆, Teff and [Fe/H] to a grid of

stellar models. The stellar models were calculated using the garstec stellar evolution code (Weiss & Schlattl (2008) and the methods used to calculate the stellar model grid are described in Serenelli et al. (2013) The results of this Bayesian analysis are given in Table 5 and are shown in Fig. 6.

6. WASP-74

WASP-74 is a V = 9.7, F9 star with a metallic-ity of [Fe/H] = +0.39 ± 0.13. The transit analysis gives a mass and radius of 1.48 ± 0.12 M⊙ and

1.64 ± 0.05 R⊙. The transit log g∗of 4.20 ± 0.02

compares to a spectroscopic log g∗ of 4.39 ± 0.07.

The evolutionary comparison (Fig. 6) suggests an evolved star with an age of 3.7 ± 0.9 Gyr and a lower mass of 1.31 ± 0.06 M⊙. The

gyrochronolog-ical and lithium age estimates are lower at 2.0+1.6_−1.0 Gyr and ∼> 2 Gyr respectively.

The planet, WASP-74b, is a relatively typical hot Jupiter in a 2-d orbit, having a mass of 0.95 ± 0.06 MJup and a moderately bloated radius of

Table 5: Bayesian mass and age estimates for the host stars Columns 2 and 3 give the maximum-likelihood estimates of the age and mass, respec-tively. Columns 4 and 5 give the mean and stan-dard deviation of the posterior age and mass dis-tribution, respectively. Star τb Mb hτ⋆i hM⋆i [Gyr] [M⊙] [Gyr] [M⊙] WASP-74 3.5 1.32 3.7 ± 0.9 1.31 ± 0.06 WASP-83 6.3 1.01 7.1 ± 2.9 1.00 ± 0.05 WASP-89 14.9 0.81 12.5 ± 3.1 0.84 ± 0.04 WASP-89a _{1.8 0.91 5.1 ± 3.3 0.87 ± 0.04} a _{Assuming α} MLT= 1.22 7

Fig. 6.— Mean stellar density versus effective tem-perature for WASP-74 (red), WASP-83 (green) and WASP-89 (blue). The best-fit isochrones (color-coded solid lines) are at 3.7 Gyr (WASP-74), 7.1 Gyr (WASP-83) and 5.1 Gyr (WASP-89). The stellar evolution tracks (color-coded dashed lines) are for masses 1.31 M⊙ (WASP-74), 1.00

M⊙ (WASP-83) and 0.87 M⊙ (WASP-89). Lines

for each star were interpolated from our grid of garstec models with α_MLT= 1.78 using the pa-rameters given in Table 5 and the appropriate value of [Fe/H] for each star. The dotted line shows an isochrone at an age of 0.1 Gyr at the same values of [Fe/H].

7. WASP-83

WASP-83 is a fainter G8 star of V = 12.9, with a metallicity of [Fe/H] = +0.29 ± 0.12. The spec-troscopic log g of 4.34 ± 0.08 is compatible with the transit log g of 4.44+0.02_−0.04. The mass of 1.11 ± 0.06 M⊙ from the transit analysis is in line with

the evolutionary estimate of 1.00 ± 0.05 M⊙. The

evolutionary age of 7.1 ± 2.9 Gyr is in line with the lithium age of ∼> 5 Gyr while the v sin i of < 0.5 km s−1_{also implies an old star.}

The planet, WASP-83b, has a mass of 0.30 ± 0.03 MJup, matching that of Saturn, and a

moder-ately bloated radius of 1.04+0.08−0.04 RJup. It is very

similar to WASP-21b (Bouchy et al. 2010), which has a similar mass (0.3 MJup), is also bloated (1.2

RJup), and also has a 4-d orbit around a G star.

8. WASP-89

With a magnitude of V = 13.1, WASP-89 is among the faintest planet-hosts found by WASP-South, but is among the more interesting systems. The spectroscopy (with a low S/N owing to the faintness) reports it as a K3 star with log g = 4.31 ± 0.16 and a mass of 0.88 ± 0.08 M⊙. The transit

analysis gives log g = 4.52 ± 0.02, with a mass of 0.92 ± 0.08 M⊙.

The initial Bayesian evolutionary analysis of WASP-89 (Table 5) gave an age of 12.5 ± 3.1 Gyr, which would likely make it older than the Galactic disk. This raises the possibility that this star is affected by the “radius anomaly” observed in many other late-type stars, particularly those like WASP-89 that show signs of magnetic ac-tivity (Hoxie 1973; Popper 1997; L´opez-Morales 2007; Spada et al. 2013). It has been proposed that this is due to the reduction in the efficiency of energy transport by convection, a phenomenon that can be approximated by reducing the mix-ing length parameter used in the model (Feiden & Chaboyer 2013; Chabrier, Gallardo & Baraffe 2007). The mixing length parameter used to cal-culate our model grid is αMLT = 1.78. With

this value of αMLT garstec reproduces the

ob-served properties of the present day Sun assuming that the composition is that given by Grevesse & Sauval (1998), the overall initial metallicity is Z = 0.01876, and the initial helium abundance is 8

Y = 0.269. There is currently no objective way to select the correct value of αMLT for a

magnet-ically active star other than to find the range of this parameter that gives plausible results. Ac-cordingly, we also calculated a Markov chain for the observed parameters of WASP-89 using stel-lar models with αMLT= 1.22, for which value we

find p(τ⋆ < 10 Gyr) = 0.91. By comparing the

results for WASP-89 with the two values of αMLT

we arrive at a mass of 0.85 ± 0.05 M⊙ with the

age being indeterminate. This is then compatible with the masses from the spectral analysis and the transit analysis.

8.1. Magnetic activity

WASP-89 shows clear evidence of magnetic ac-tivity in the form of a rotational modulation and through star-spots during transit. Three years of WASP-South data all show a ∼ 1 per cent modu-lation at a period near 20 d, and the fourth shows a modulation at half that (10 d), presumably the first harmonic of the rotational period caused by a more complex spot pattern (Table 6, Fig. 7). The average from four different years of WASP-South data is a rotational period of Prot = 20.2 ± 0.4 d.

This, together with our value for the stellar radius, implies a value of Vrot = 2.2 ± 0.1 km s−1, which

compares to the spectroscopic Vrotsin I estimate

of 2.5 ± 0.9 km s−1_{. This is consistent with the}

WASP-89’s spin axis being at 90◦ _{to us.}

The transit lightcurves from TRAPPIST and EulerCAM show clear star spots, visible as bumps in the transit profile, most clearly at phase 0.997 in the EulerCAM lightcurve from the 02 Oct 2012 (Fig. 4). This raises the issue of whether we are treating the lightcurves correctly in our MCMC

Table 6: Periodogram analysis of the WASP lightcurves for WASP-89. Observing dates are JD – 2450 000, N pts is the number of data points, Amp is the semi-amplitude (in magnitudes) of the best-fit sine wave at the period P found in the periodogram with false-alarm probability FAP.

Year Dates Npts P [d] Amp FAP

2008 4622–4752 6648 20.69 0.007 0.006 2009 4970–5116 5589 10.46 0.006 0.072 2011 5691–5858 4072 19.65 0.014 <0.001 2012 6053–6107 1430 19.57 0.010 0.004

Fig. 7.— Periodograms of the WASP lightcurves for WASP-89 obtained in 2008 (top left) and 2009 (bottom left). Horizontal lines indicate false-alarm probability levels of 0.1, 0.01 and 0.001. Top right shows the 2008 data folded on 20.69 d; bottom right the 2009 data folded in 10.46 d.

analysis (See the discussion in, e.g., Oshagh et al. 2013). When a planet transits a spot we see a slight brightening, and including such data will cause the fitted transit to be shallower. However, any spots that are not transited but still present will do the opposite, causing the transit to be deeper. Thus excluding bumps caused by tran-sited spots would introduce a bias. Without more information on the extent of spottedness there is no secure way of dealing with this. The rotation modulation suggests that the difference between different faces of the star is of order 1 per cent of the brightness, which is comparable to other un-certainties. We have thus chosen to simply com-bine all the lightcurves in the analysis, effectively averaging over any spots present.

In principle one can use transits of star spots to deduce the orbital alignment (e.g. Tregloan-Reed, Southworth & Tappert 2013). The TRAPPIST and EulerCAM lightcurves from 12 Sep 2012 are of the same transit, as are those from 02 Oct 2012, the latter being 6 orbital cycles (20.1 d) later. There appears to be a spot at phase 0.992 in the 12 Sep lightcurve and a spot at 0.997 in the 02 Oct lightcurve. This could be the same spot being transited one stellar rotation later, which is un-9

stellar rotation (or anti-aligned).

If it is the same spot, the difference in the phase of the spot transit implies that the star had ro-tated by 1.07 cycles (or 0.93 cycles), which trans-lates to a rotation period of 18.8 ± 0.3 d (or 21.7 ± 0.3 d). This is slightly different from the value of 20.2 ± 0.4 d from the WASP data, but the dis-crepancy might be accounted for by differential rotation.

Thus, we conclude that WASP-89 rotates with a 20-d period and is magnetically active, and that there are indications that the planetary orbit is aligned or anti-aligned. However, we need more extensive star-spot observations and observations of the Rossiter–McLaughin effect to be sure. 8.2. A massive planet in an eccentric orbit

WASP-89b has a mass of 5.9 ± 0.4 MJup and

is in a 3.356-d orbit with an eccentricity of 0.19 ± 0.01. It thus joins a small number of massive plan-ets in short-period, eccentric orbits, of which the most similar are XO-3b (12 MJup, 3.2 d, e = 0.26;

Johns-Krull et al. 2008), HAT-P-2b (8.7 MJup, 5.6

d, e = 0.52; Bakos et al. 2007) and HAT-P-21b (4.0 MJup, 4.1 d, e = 0.23; Bakos et al. 2011).

It is worth noting, though, that those three planets orbit stars of spectral type F5, F8 and G3, respectively. WASP-89 is the first known K star hosting a massive planet in a short-period eccen-tric orbit (M > 1 MJup; P < 6 d; e > 0.1). The

magnetic activity of WASP-89 could be related to the hosting of a massive, short-period planet, since magnetic activity might be enhanced in hot-Jupiter hosts (e.g. Poppenhaeger & Wolk 2014).

Planets in eccentric, short-period orbits are of particular interest in that their rotation cannot be fully phase-locked to their orbit, and so they must experience large differences in radiative forcing around the orbit. Thus they can tell us about the dynamics of giant-planet atmospheres (e.g. Wong et al. 2014 and references therein).

The usual explanation for the occurrence of such eccentric orbits in short-period hot Jupiters is that they are moved in inwards by a process of “high eccentricity migration”, followed by cir-cularisation (e.g. Rasio & Ford 1996; Fabrycky & Tremaine 2007; Naoz et al. 2011; Socrates et al. 2012a).

from (Adams & Laughlin 2006, eqn 3):

τcir ≈ 1.6 Gyr ×  QP 106  × MP MJup  ×M∗ M⊙ −3/2 × RP RJup −5 × a 0.05 AU
13/2

The value of the quality factor, QP, is unclear,

but if we take it as 105_{(e.g. Socrates, Katz & Dong}

2012b), then we obtain for WASP-89b a circulari-sation timescale of ∼ 2 Gyr. Here, the large mass of the planet prevents circularisation in less than a Gyr despite the short orbit. This timescale is in line with the gyrochronological age of the host star, and thus the fact that the planet has not circularised is consistent. Tidal damping of eccen-tricity is expected to occur faster than damping of obliquity or inwards orbital decay (Matsumura, Peale & Rasio 2010), and thus we would expect the current values of these properties to be direct products of the high-eccentricity migration. Acknowledgements

WASP-South is hosted by the South African Astronomical Observatory and we are grateful for their ongoing support and assistance. Fund-ing for WASP comes from consortium universi-ties and from the UK’s Science and Technology Facilities Council. The Euler Swiss telescope is supported by the Swiss National Science Foun-dation. TRAPPIST is funded by the Belgian Fund for Scientific Research (Fond National de la Recherche Scientifique, FNRS) under the grant FRFC 2.5.594.09.F, with the participation of the Swiss National Science Fundation (SNF). This pa-per includes observations made with the RISE photometer on the 2.0-m Liverpool Telescope un-der PATT program PL14A10. The Liverpool Tele-scope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Fa-cilities Council. M. Gillon and E. Jehin are FNRS Research Associates. A.H.M.J. Triaud is a Swiss National Science Foundation Fellow under grant P300P2-147773. L. Delrez acknowledges the sup-port of the F.R.I.A. fund of the FNRS.

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This 2-column preprint was prepared with the AAS LA_TEX

macros v5.2.

BJD – 2400 000 RV σRV Bisector (UTC) (km s−1_{) (km s}−1_{) (km s}−1₎ WASP-74: 55795.60542 −15.8781 0.0045 −0.0067 55796.64971 −15.6394 0.0047 −0.0094 55802.72044 −15.6772 0.0056 0.0026 55820.55636 −15.7453 0.0042 0.0028 55823.52316 −15.8463 0.0060 0.0014 55824.54772 −15.6601 0.0045 −0.0213 55826.57775 −15.6553 0.0049 −0.0019 55827.50202 −15.8600 0.0054 −0.0132 55828.57525 −15.6596 0.0051 −0.0147 55829.63956 −15.8690 0.0044 0.0028 55830.50481 −15.6975 0.0050 −0.0227 55832.53917 −15.7364 0.0042 0.0048 55834.53461 −15.7850 0.0042 −0.0112 55835.53651 −15.7357 0.0050 −0.0019 55851.58947 −15.7856 0.0048 0.0015 56049.91441 −15.9043 0.0041 −0.0147 56103.75557 −15.8386 0.0039 −0.0158 56150.65595 −15.8523 0.0051 −0.0261 56152.55759 −15.8601 0.0052 −0.0277 56212.54690 −15.8647 0.0042 0.0007 WASP-83: 55626.85323 31.5152 0.0160 −0.0290 55648.77839 31.5828 0.0120 0.0340 55651.73866 31.4915 0.0162 −0.0095 55675.76117 31.5583 0.0176 0.0029 55676.66336 31.5096 0.0132 0.0412 55677.75049 31.5340 0.0123 0.0106 55679.65881 31.5543 0.0109 0.0237 55680.67015 31.5312 0.0148 0.0003 55682.61842 31.5544 0.0081 −0.0127 55683.69704 31.5697 0.0093 0.0069 55684.70318 31.5667 0.0100 0.0015 55707.63183 31.5492 0.0114 0.0389 55711.55909 31.4847 0.0194 0.0092 55721.53003 31.5056 0.0117 −0.0133 55767.53761 31.5751 0.0141 0.0194 55768.48367 31.5820 0.0133 0.0048 55769.49626 31.5754 0.0092 0.0221 55952.79597 31.5772 0.0083 0.0112 55958.80682 31.5390 0.0089 −0.0186 55978.72739 31.5360 0.0131 0.0089 55981.69825 31.5742 0.0148 −0.0285 55982.78430 31.5790 0.0094 −0.0113 55983.85735 31.5348 0.0091 −0.0091 55984.67552 31.5218 0.0097 0.0191 55985.69305 31.5433 0.0097 0.0102 56030.67402 31.5659 0.0136 0.0052 56038.67878 31.5257 0.0149 −0.0179 56336.84838 31.5461 0.0115 0.0185

Bisector errors are twice RV errors

BJD – 2400 000 RV σRV Bisector (UTC) (km s−1_{) (km s}−1_{) (km s}−1₎ WASP-89: 55685.90591 21.8497 0.0245 −0.0109 56123.72535 20.4472 0.0315 −0.0556 56124.69378 20.6905 0.0303 −0.0124 56125.75998 22.0629 0.0296 −0.0349 56126.68173 20.8330 0.0242 0.1185 56136.84966 20.7044 0.0486 0.1745 56150.73985 20.4600 0.0794 0.0443 56151.77120 20.9629 0.0503 0.1013 56154.55728 20.4647 0.0630 0.1552 56158.71898 21.2050 0.0527 0.0643 56159.55775 21.9642 0.0629 0.1241 56159.78044 21.7811 0.0685 0.1462 56181.67023 20.6210 0.0240 −0.0228 56182.64854 21.8869 0.0278 0.0953 56184.60865 20.4188 0.0268 0.0246 56186.53071 21.9080 0.0464 0.0243 56187.54974 20.5000 0.0723 0.0951 56203.56053 21.4453 0.0526 0.0432 56212.57303 21.4703 0.0364 −0.0872 56419.82154 20.5757 0.0407 0.0355 Bisector errors are twice RV errors